Gunma mountain was quiet for 3,000 years before sudden eruption

KUSATSU, GUNMA PREF. – Fresh details emerged Wednesday about the volcanic eruption at Mount Moto-Shirane in Gunma Prefecture and how it caught the Meteorological Agency and volcanic experts unaware, prompting the agency to release a delayed volcanic warning.

Commenting on Tuesday morning’s sudden eruption, Makoto Saito, director of the agency’s volcanology division, told a news conference later in the day that because there had been “no observational data that suggested signs of volcanic activity before the eruption,” raising the volcanic alert level beforehand would have been near impossible.

According to the agency, there had been no volcanic activity at the Kagamiike crater, the apparent site of the eruption, for about 3,000 years until Tuesday. Because of this long period of dormancy, it had not been part of the agency’s 24/7 volcano-monitoring program.

On the other hand, Mount Moto-Shirane’s Yugama crater, located approximately 2 km north of Kagamiike crater, had been under 24-hour surveillance, with volcanic activity being observed as recently as 2011.

Currently, 50 out of 111 active volcanoes nationwide are under constant watch by the agency. The 50 are selected by a team of experts in coordination with the agency, and recent activity is a crucial factor in deciding whether they are continuously monitored, agency official Toshihiro Imataki told The Japan Times.

Even so, craters that are not active are surveyed once a year by mobile observation teams, he added.

Takahiro Yamamoto, chief researcher at the National Institute of Advanced Industrial Science and Technology, said Tuesday’s eruption “happened in a place we didn’t expect. We can’t be sure of what might happen next.”

After analyzing the eruption’s aftermath, including the volcanic ash that blanketed a nearby ski resort, Kenji Nogami, a professor at the Tokyo Institute of Technology, confirmed Wednesday that the event appeared to have been “a typical phreatic eruption,” or a steam-driven explosion caused when water is heated beneath the ground. Such explosions are more difficult to detect beforehand compared to eruptions caused by the explosion of lava.

Nogami said that in order to detect such eruptions, there was a “need to monitor the craters more thoroughly by installing equipment such as seismometers.”

While the number of volcanic earthquakes has fallen significantly since the eruption at 9:59 a.m. Tuesday, the agency warned that the 2,171-meter mountain, which is part of Mount Kusatsu-Shirane, could again erupt, spewing large volcanic rocks, ash deposits and volcanic gases.

On Wednesday, the land ministry sent experts to survey the eruption by helicopter, while local police and firefighters were searching the area to make sure there were no victims left behind. Those operations, however, were later halted after continuing volcanic tremors were detected.

Yasuo Ishizaki, a volcanic geology professor at the University of Toyama, said that although the likelihood of volcanic mud flow, which often accompanies a snowy mountain eruption, occurring at the mountain appears to be low, “if a large-scale eruption accompanying excretion of lava occurs, it would increase the risk.”